Shock absorbers are used to reduce force transmitted to a structure such as a vehicle chassis, known as a ‘sprung part’, from a moving part such as a wheel, known as an ‘unsprung part’.
A conventional vehicle shock absorber comprises a body connected to the chassis of the vehicle, the body having a bore which contains a volume of working fluid. A piston is connected to the wheel and arranged within and axially displaceable along the bore, dividing the bore into a ‘compression chamber’ and a ‘rebound chamber’. When the wheel comes into contact with irregular portions of road or obstacles it is displaced, forcing the piston through the bore. Due to the working fluid within the bore, the movement of the piston is obstructed and the rate of displacement is dependent on vents through the piston allowing the working fluid to pass through. This action reduces the velocity of the piston and dissipates kinetic energy, known as providing a ‘damping force’.
Shock absorbers are typically ‘velocity sensitive’ and generate damping force as a function of shaft velocity. Some shock absorbers are also ‘position sensitive’ and generate damping force as a function of shaft velocity and piston location (position) within the bore. This provides an additional benefit that damping force may be varied according to amplitude of piston displacement. This typically involves providing a low damping force for small amplitude inputs and a larger damping force for large amplitude inputs.
One type of position sensitive shock absorber is known as a ‘bypass shock absorber’ which has an auxiliary conduit, known as a ‘bypass passage’, that communicates the working fluid between two locations in the bore during a defined portion of the stroke of the piston, known as a ‘bypass zone’. When the piston is displaced in this portion, the bypass passage allows additional working fluid to be displaced between the two locations and increases the rate of fluid flowing past the piston, which decreases the damping force. This is useful where, for example, during general use (such as on-road use) the piston is typically displaced in a mid-portion of the bore in a bypass zone, and a low damping force is required. However, when the vehicle is used off-road and typically encounters larger obstacles and impact forces, this displaces the piston out of the bypass zone, substantially increasing the damping force and absorbing the impact force.
One example of a bypass shock absorber is disclosed in U.S. Pat. No. 7,191,877 in the name of Thyssenkrupp Bilstein of America, Inc. in which a body houses a bore and four bypass passages, the bypass passages having different effective lengths and being operable over a different section of the bore. During part of either a compression or rebound stroke of a piston within the bore, there are either two, one or no bypass passages communicating fluid from one side of a piston to the other, progressively increasing the damping force as less bypass passages communicate fluid. Each bypass passage has an adjustable one-way ‘check valve’ located at one end of the passage to prevent fluid from flowing through the passage during a defined stroke direction. This has the effect that two of the bypass passages communicate fluid around the piston in a compression stroke only and two allow fluid to pass in a rebound stroke only. Each of the adjustable valves are adjustable to control the rate of fluid flowing through each valve. The valve is adjusted by rotating a dial which changes the size of an aperture (orifice) fluid can flow through, thereby restricting the flow of fluid through each bypass passage and increasing the damping force provided.
Whilst this would provide an adequate position sensitive shock absorber, the disclosed shock absorber also has a number of disadvantages. For example, if a user wishes to tune the damping characteristics of the shock absorber, each adjustable valve must be individually adjusted. If the user wishes for the tuning of the bypass passages to be consistent, each valve must be individually adjusted to provide a consistent damping characteristic. This is not only time consuming but also complex for the user to perform accurately, as each valve of this type has a non-linear flow characteristic. The non-linear flow characteristic means that the valve offers little resistance to flow at low fluid flow rates but substantial resistance at higher fluid flow rates. This results in little damping force being provided for low velocity impacts, which is typical during on-road use, and significantly more for high velocity impacts, resulting in an unnecessarily soft ride and poor chassis control when used on-road.
To counteract the poor on-road handling provided by this style of shock absorber, the shock absorber is commonly used in conjunction with a secondary, coil-carrying shock absorber to provide additional, low-velocity damping. However this further increases the inconvenience and complexity of this style of shock absorber, requiring more space in the vehicle and increasing the likelihood of maintenance issues.
Accordingly, it would be useful to provide a bypass shock absorber which provides adequate low-velocity and high-velocity damping, without requiring a secondary shock absorber. This is of particular importance in relation to retro-fitting such a shock absorber, as many vehicle typically provide space for a single shock absorber to be fitted to each wheel. Furthermore, it would be advantageous to provide such a shock absorber which can be conveniently and accurately adjusted to provide a range of damping force.